I read your question at least 6 times, and I don't understand the problem. Would you be so kind to clarify a bit? When I read the answers, I'm even more confused ;)
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user2567Jan 19 '11 at 18:48

To be honest, a lot depends on how you track project completion. Let's face it, some tasks have to be done before others. If you don't get that prerequisite task done, others will fail in turn. In this case, it would be better to finish that task--spanning a sprint if necessary. In either case, keep the team in the loop on problems you're facing at the daily standup. The team lead will make the final determination of when/if the item gets deferred.
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Berin LoritschJan 19 '11 at 18:57

3 Answers
3

You may find that a particular high risk task encountered problems on day 1 of the sprint. You know you're going to fail, so flagging it to your team lead early on and putting it into the backlog means you've failed fast. You can then carry on with other items while others tackle the problems that caused this task to take too long.

And always remember to contact Product Owner. It is his sole responsibility to react and take decisions regarding fast-failed tasks. You can put it to backlog, split it before next sprint, and take other smaller tasks during this sprint. Just ask Product Owner :)
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Marcin GilJan 19 '11 at 20:00

I'd say get rid of the sprint all together But if you're stuck with it, the reason it's better to defer right away is so that the customer can expect it rather than hearing on the last day "oh we didn't do feature x, y, or z"

This is a communication issue. As a developer you'll want to notify management or development lead that you have impediments or things blocking your ability to deliver quickly.

The greatest advantage to communicating these issues early and often are that you can work with your team to 'swarm' the issue and resolve it quickly, or defer it for another sprint or development build.